kgo: pin AddPartitionsToTxn to v3 when using one transaction

KIP-890 has been updated such that v3 must be used by clients. We will
pin to v3 unless multiple transactions are being added, or unless any
transaction is verify only.

Closes #609.

pkg/kgo/client.go

@@ -3420,10 +3420,17 @@ func (cl *addPartitionsToTxnSharder) shard(ctx context.Context, kreq kmsg.Reques
 
 	var issues []issueShard
 	for id, req := range brokerReqs {
-		issues = append(issues, issueShard{
-			req:    req,
-			broker: id,
-		})
+		if len(req.Transactions) <= 1 || len(req.Transactions) == 1 && !req.Transactions[0].VerifyOnly {
+			issues = append(issues, issueShard{
+				req:    &pinReq{Request: req, pinMax: true, max: 3},
+				broker: id,
+			})
+		} else {
+			issues = append(issues, issueShard{
+				req:    req,
+				broker: id,
+			})
+		}
 	}
 	for _, unkerr := range unkerrs {
 		issues = append(issues, issueShard{